The Sertoli cell, the only nongerminal element within the seminiferous epithelium of the testis, is believed to provide a nurse-like function to the developing germ cells during spermatogenesis. This proposal seeks support to study a growth factor secreted by Sertoli cells in culture (Sertoli cell secreted growth factor, SCSGF). We have evidence that Sertoli cell conditioned medium contains at least two (one of which is EGF- like) and possibly more mitogens which stimulate the proliferation of cell lines of epidermal, mesodermal and endothelial origin. Based on biochemical studies, this growth factor, is distinct from the previously identified testicular growth factors. The goals of this proposal are to purify and characterize SCSGF from Sertoli cell conditioned medium, to identify the testicular target cells for this novel growth factor, and to compare this growth factor with other previously identified growth factors. Although the rat will be the model system for these studies, we have access to human tissues and the studies will include experiments on the human Sertoli cell mitogen. A growth assay using one of several cell lines will be used to measure mitogenic activity during purification. Because of the importance role of the Sertoli cell in the regulation of spermatogenesis, we are convinced that growth factor secretion by the Sertoli cell is required for normal testicular function. We will test the effect of the mitogen, both in vivo and in vitro, on germ cell proliferation and stem cell renewal, as well as to investigate the effect of this mitogen on other testicular cell types. Purified radiolabeled SCSGF will be used in binding assays with testicular sections to identify the cell types which specifically bind SCSGF. Additionally, we will investigate SCSGF secretion in vivo and in vitro during the spermatogenic cycle. The experiments are designed to use very small amounts of SCSGF. The SCSGF may have great therapeutic applications for individuals whose testes have been depleted of germ cells. Thus, we will investigate the exciting possibility that this mitogen plays a role in spermatogenesis.